1. Field of Invention
The invention relates to improved catalysts, methods of making the same and methods of using the same in a process for low temperature oxydehydrogenation of ethane to acetic acid without or having reduced production of ethylene and CO by-products in the product stream based on the composition of the catalyst. The invention also relates to a one step vapor phase catalytic process using the novel catalyst featuring increased ethane conversion and higher selectivity to acetic acid up to 80% at particular process conditions.
2. Description of Related Art
Several publications are referenced in this application. These references describe the state of the art to which this invention pertains, and are hereby incorporated by reference.
Utilization of lower alkane (C1-C4) as feed stock to produce added value petrochemicals is an industrially desired process. Lower alkanes are low cost and environmentally acceptable because of their low chemical reactivity. There are only few commercially available chemical catalytic processes which utilize lower alkane as a feed, such as butane to maleic anhydride.
Acetic acid is conventionally produced by methanol carbonylation using expensive rhodium catalysts in a liquid phase homogeneous reaction. This requires complicated procedures for recovery of the catalyst and isolation of products. More recently, acetic acid has been produced from an expensive raw material, ethylene, with the production of acetaldehyde as a by-product.
The use of molybdenum and vanadium-containing catalyst systems for low temperature oxydehydrogenation of ethane to ethylene was reported by E. M. Thorsteinson et.al., Journal of Catalysts, vol. 52, pp. 116-132 (1978). This paper discloses mixed oxide catalysts containing molybdenum and vanadium together with another transition metal oxide, such as Ti, Cr, Mn, Fe, Co, Ni, Nb, Ta, or Ce. The catalysts are active at temperatures as low as 200xc2x0 C. for the oxydehydrogenation of ethane to ethylene. Some acetic acid is produced as a by-product.
Several U.S. patents (U.S. Pat. Nos. 4,250,346, 4,524,236, 4,568,790, 4,596,787 and 4,899,003) have been granted on low temperature oxydehydrogenation of ethane to ethylene. U.S. Pat. No. 4,250,346 discloses the use of catalysts of the formula MohViNbjAk in which A is Ce, K, P, Ni, and/or U, h is 16, i is 1 to 8, j is 0.2 to 10, and k is 0.1 to 5. U.S. Pat. No. 4,454,236 is directed to the use of a calcined catalyst of the formula MoaVbNbcSbdXe.
The above cited patents make reference to other patents concerned with the production of ethylene from ethane by the oxydehydrogenation process and all make reference to the formation of acetic acid as a by-product.
U.S. Pat. Nos. 4,339,355 and 4,148,757 disclose oxide catalysts containing Mo, Nb, V and a fourth metal selected from Co, Cr, Fe, In, Mn or Y for the oxidation/ammoxidation of unsaturated aliphatic aldehyde to corresponding saturated aliphatic carboxylic acids.
European Patent Publication EP 02 94 845 discloses a process for the higher selective production of acetic acid by the oxidation of ethane with oxygen in contact with a mixture of catalysts consisting of (A) a catalyst for oxydehydrogenation of ethane to ethylene and (B) a catalyst for hydration/oxidation of ethylene. The ethane oxydehydrogenation catalyst is represented by the formula MoxVyZz, wherein Z can be one or more of the metals Nb, Sb, Ta, Ca, Sr, Ti and W. The catalyst for hydration/oxidation is selected from a molecular sieve catalyst, a palladium-containing oxide catalyst, tungsten-phosphorus oxides, or a tin molybdenum containing oxide catalysts. European Patent Publication EP 02 94 845 employs a catalyst prepared by a physical mixture of two types of catalysts. This patent does not disclose the catalyst of the present invention which is designed in such a way that it has both oxydehydrogenation and oxygenation properties on the same catalyst.
European Patent Publication EP 04 80 594 is directed to the use of an oxide catalyst composition comprising tungsten, vanadium, rhenium and at least one of the alkaline metals for the production of ethylene and acetic acid by oxidation of ethane with a molecular oxygen-containing gas. The replacement of tungsten in whole or part by molybdenum carried out in European Patent Publication EP 04 07 091 results in an increase in selectivity to acetic acid at the expense of the selectivity to ethylene.
European Patent Publication EP 05 18 548 is concerned with a process for the production of acetic acid by ethane oxidation in contact with a solid catalyst having an empirical formula VPaMbOx, where M is one or more optional elements selected from Co, Cu, Re, Nb, W and many other elements, excluding molybdenum, a is 0.5 to 3, b is 0 to 0.1.
European Patent Publication EP 06 27 401 describes the use of a VaTibOx catalyst for the oxidation of ethane to acetic acid. The catalyst composition may comprise additional components from a large list of possible elements. The reference does not disclose any examples of catalysts comprising those elements in combination with vanadium, titanium and oxygen. Further, recently reported catalysts containing MoVNb promoted with phosphorus can produce a relatively higher yield of acetic acid as compared to unpromoted catalyst with the production of by products such as carbon monoxide, carbon dioxide and ethylene (U.S. Pat. No. 6,013,597, filed Sep. 17, 1997, entitled xe2x80x9cCatalysts for the Oxidation of Ethane to Acetic Acid, Processes of Making Same and Processes of Using the Samexe2x80x9d, hereby incorporated by reference). Further, due to environmental law constraints, carbon monoxide is a less desirable by-product.
Accordingly, it would be desirable to produce an improved catalyst for use in the selective production of acetic acid from ethane through a single stage partial oxidation process without the production of carbon monoxide and ethylene.
According to the present invention, ethane is oxidized with molecular oxygen to acetic acid in a gas phase reaction at relatively high levels of conversion, selectivity and productivity and at temperatures ranging from 150xc2x0 C. to 450xc2x0 C. and at pressures ranging from 1-50 bar. This is achieved using a catalyst having a calcined composition of MoaVbNbcPdd, wherein:
a is 1 to 5;
b is 0 to 0.5;
c is 0.01 to 0.5; and
d is 0 to 0.2.
The numerical values of a, b, c and d represent the relative gram-atom ratios of the elements Mo, V, Nb and Pd, respectively, in the catalyst. The elements are present in combination with oxygen in the form of various oxides. The inventive catalysts are preferably produced using the methods disclosed herein.